With rapid spread of information-related equipment or communications equipment such as personal computers, video cameras, and mobile phones in recent years, development of batteries used as a power source thereof has been considered to be important. Among these batteries, a lithium ion battery has attracted attention from a viewpoint of high energy density.
In the lithium ion battery that is currently commercially available, a liquid electrolyte containing a flammable organic solvent is used, and therefore, attachment of a safety device that suppresses an increase in temperature at the time of short circuit, or improvement in structural and material aspects for preventing short circuit is required. On the other hand, in the lithium ion battery formed into all-solid-state battery by using a solid electrolyte in place of the liquid electrolyte, it is considered that the flammable organic solvent is not used in the battery, and therefore, the safety device can be simplified, and the lithium ion battery is excellent in a production cost or productivity.
As the solid electrolyte used in the lithium ion battery, a sulfide solid electrolyte is known. As a crystal structure of the sulfide solid electrolyte, various structures are known. As one of such crystal structures, an argyrodite-type crystal structure is known (Patent Literatures 1 to 5, Non-Patent Literatures 1 to 3). Non-Patent Literatures 4 and 5 disclose a solid electrolyte that has a composition of Li6PS5X and contains chlorine and bromine at a specific ratio. Some of the argyrodite-type crystal structures have high lithium ion conductivity. However, further improvement in ion conductivity is required. In addition, in general, the sulfide solid electrolyte has a problem of a possibility of reacting with moisture in air to generate hydrogen sulfide.